[PATCH AUTOSEL 6.13 06/21] futex: Pass in task to futex_queue()

Sasha Levin
2025-02-11T01:30:04Z

From: Jens Axboe <axboe@kernel.dk>

[ Upstream commit 5e0e02f0d7e52cfc8b1adfc778dd02181d8b47b4 ]

futex_queue() -> __futex_queue() uses 'current' as the task to store in
the struct futex_q->task field. This is fine for synchronous usage of
the futex infrastructure, but it's not always correct when used by
io_uring where the task doing the initial futex_queue() might not be
available later on. This doesn't lead to any issues currently, as the
io_uring side doesn't support PI futexes, but it does leave a
potentially dangling pointer which is never a good idea.

Have futex_queue() take a task_struct argument, and have the regular
callers pass in 'current' for that. Meanwhile io_uring can just pass in
NULL, as the task should never be used off that path. In theory
req->tctx->task could be used here, but there's no point populating it
with a task field that will never be used anyway.

Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/22484a23-542c-4003-b721-400688a0d055@kernel.dk
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
io_uring/futex.c | 2 +-
kernel/futex/core.c | 5 +++--
kernel/futex/futex.h | 11 ++++++++---
kernel/futex/pi.c | 2 +-
kernel/futex/waitwake.c | 4 ++--
5 files changed, 15 insertions(+), 9 deletions(-)

diff --git a/io_uring/futex.c b/io_uring/futex.c
index e29662f039e1a..f108da4ff863c 100644
--- a/io_uring/futex.c
+++ b/io_uring/futex.c
@@ -349,7 +349,7 @@ int io_futex_wait(struct io_kiocb *req, unsigned int issue_flags)
hlist_add_head(&req->hash_node, &ctx->futex_list);
io_ring_submit_unlock(ctx, issue_flags);

- futex_queue(&ifd->q, hb);
+ futex_queue(&ifd->q, hb, NULL);
return IOU_ISSUE_SKIP_COMPLETE;
}

diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index ebdd76b4ecbba..3db8567f5a44e 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -532,7 +532,8 @@ void futex_q_unlock(struct futex_hash_bucket *hb)
futex_hb_waiters_dec(hb);
}

-void __futex_queue(struct futex_q *q, struct futex_hash_bucket *hb)
+void __futex_queue(struct futex_q *q, struct futex_hash_bucket *hb,
+ struct task_struct *task)
{
int prio;

@@ -548,7 +549,7 @@ void __futex_queue(struct futex_q *q, struct futex_hash_bucket *hb)

plist_node_init(&q->list, prio);
plist_add(&q->list, &hb->chain);
- q->task = current;
+ q->task = task;
}

/**
diff --git a/kernel/futex/futex.h b/kernel/futex/futex.h
index 99b32e728c4ad..6b2f4c7eb720f 100644
--- a/kernel/futex/futex.h
+++ b/kernel/futex/futex.h
@@ -285,13 +285,15 @@ static inline int futex_get_value_locked(u32 *dest, u32 __user *from)
}

extern void __futex_unqueue(struct futex_q *q);
-extern void __futex_queue(struct futex_q *q, struct futex_hash_bucket *hb);
+extern void __futex_queue(struct futex_q *q, struct futex_hash_bucket *hb,
+ struct task_struct *task);
extern int futex_unqueue(struct futex_q *q);

/**
* futex_queue() - Enqueue the futex_q on the futex_hash_bucket
* @q: The futex_q to enqueue
* @hb: The destination hash bucket
+ * @task: Task queueing this futex
*
* The hb->lock must be held by the caller, and is released here. A call to
* futex_queue() is typically paired with exactly one call to futex_unqueue(). The
@@ -299,11 +301,14 @@ extern int futex_unqueue(struct futex_q *q);
* or nothing if the unqueue is done as part of the wake process and the unqueue
* state is implicit in the state of woken task (see futex_wait_requeue_pi() for
* an example).
+ *
+ * Note that @task may be NULL, for async usage of futexes.
*/
-static inline void futex_queue(struct futex_q *q, struct futex_hash_bucket *hb)
+static inline void futex_queue(struct futex_q *q, struct futex_hash_bucket *hb,
+ struct task_struct *task)
__releases(&hb->lock)
{
- __futex_queue(q, hb);
+ __futex_queue(q, hb, task);
spin_unlock(&hb->lock);
}

diff --git a/kernel/futex/pi.c b/kernel/futex/pi.c
index d62cca5ed8f4c..635c7d5d42220 100644
--- a/kernel/futex/pi.c
+++ b/kernel/futex/pi.c
@@ -982,7 +982,7 @@ int futex_lock_pi(u32 __user *uaddr, unsigned int flags, ktime_t *time, int tryl
/*
* Only actually queue now that the atomic ops are done:
*/
- __futex_queue(&q, hb);
+ __futex_queue(&q, hb, current);

if (trylock) {
ret = rt_mutex_futex_trylock(&q.pi_state->pi_mutex);
diff --git a/kernel/futex/waitwake.c b/kernel/futex/waitwake.c
index 3a10375d95218..a9056acb75eef 100644
--- a/kernel/futex/waitwake.c
+++ b/kernel/futex/waitwake.c
@@ -350,7 +350,7 @@ void futex_wait_queue(struct futex_hash_bucket *hb, struct futex_q *q,
* access to the hash list and forcing another memory barrier.
*/
set_current_state(TASK_INTERRUPTIBLE|TASK_FREEZABLE);
- futex_queue(q, hb);
+ futex_queue(q, hb, current);

/* Arm the timer */
if (timeout)
@@ -461,7 +461,7 @@ int futex_wait_multiple_setup(struct futex_vector *vs, int count, int *woken)
* next futex. Queue each futex at this moment so hb can
* be unlocked.
*/
- futex_queue(q, hb);
+ futex_queue(q, hb, current);
continue;
}

--
2.39.5